Environmental and ecological impacts of water supplement schemes in a heavily polluted estuary

Water supplement has been used to improve water quality in a heavily polluted river with small base flow. However, its adverse impacts particularly on nearby sensitive ecosystems have not been fully investigated in previous studies. In this paper, using the Shenzhen River estuary in China as a case study, the impacts of two potential water supplement schemes (reclaimed water scheme and seawater scheme) on water quality improvement and salinity alteration of the estuary are studied. The influences of salinity alteration on the dominant mangrove species (Aegiceras corniculatum, Kandelia candel, and Avicennia marina) are further evaluated by comparing the alteration with the historical salinity data and the optimum salinity range for mangrove growth. The results obtained indicate that the targets of water quality improvement can be achieved by implementing the water supplement schemes with roughly the same flow rates. The salinity under the reclaimed water scheme lies in the range of historical salinity variation, and its average value is close to the optimum salinity for mangrove growth. Under the seawater scheme, however, the salinity in the estuary exceeds the range of historical salinity variation and approaches to the upper bound of the survival salinity of the mangrove species which have a relatively low salt tolerance (e.g. A. corniculatum). Therefore, the seawater scheme has negative ecological consequences, while the reclaimed water scheme has less ecological impact and is recommended in this study.